With the development of wireless communication technology, various digital modulation ways (e.g., 16QAM/64QAM/OFDM) of high spectral efficiency are widely used. As compared with the conventional constant envelope modulation, an envelope of the signal transmitted by the digital modulation way of high spectral efficiency has a high Peak-to-Average-Power Ratio (PAPR). When the transmitted signal passes the nonlinear amplifier, inter-modulation items will be generated. The inter-modulation items not only cause a distortion of the in-band signal, but also lead to a spectrum leakage, thus the adjacent channels are interfered with each other.
Essentially, the predistortion technology is to obtain the inverse transfer characteristic of the power nonlinear amplifier by measuring the nonlinear transfer characteristic thereof, and integrate the inverse transfer characteristic as predistortion data into a predistortion module of the power nonlinear amplifier, so as to compensate the nonlinear distortion caused when the signal passes the power nonlinear amplifier.
It is a conventional method to approximate nonlinear inverse transfer characteristic of the amplifier through a curve-fitting technology. FIG. 1 illustrates a schematic diagram of a linear approximation acquired by a curve-fitting method. As illustrated in FIG. 1, the curve may be fitted by using a plurality of segments of straight line. For example, when the values of end points u1 and u2 of each segment of straight line are known, the value of point uk within the segment of straight line may be calculated by the following Equation (1):
                              G          ⁡                      (                          u              k                        )                          =                                                                              G                  ⁡                                      (                                          u                      2                                        )                                                  -                                  G                  ⁡                                      (                                          u                      1                                        )                                                                                                u                  2                                -                                  u                  1                                                      ⁢                          (                                                u                  k                                -                                  u                  1                                            )                                +                      G            ⁡                          (                              u                1                            )                                                          (        1        )            
Theoretically, an optimal approximation of the inverse transfer characteristic of the amplifier can be implemented through infinite piecewise fitting. However, it is obvious that in the actual predistortion system, excessive segments will cause rapid increase of the circuit complexity. Thus it is ordinary to take the segment points as parameters, and obtain an optimum segment point through a search, so as to depress the increase of the circuit complexity caused by excessive segments.
FIG. 2 illustrates an exemplary block diagram of a wireless communication system including an adaptive segment digital predistortion device in the relevant art.
As illustrated in FIG. 2, the wireless communication system includes a predistorter 100, a digital-to-analog (D/A) converter 101, a quadrature modulator 102, a mixer 103, an amplifier 104, an antenna 105, a coupler 106, an attenuator 107, a mixer 108, an analog band-pass filter 109, a power detector 110, an analog-to-digital (A/D) converter 111, an averager 112, a parameter updater 113, a look-up table updater 114, a look-up table 115 and a power calculator 116.
The power calculator 116 calculates instantaneous power of an input signal of the digital baseband. The look-up table 115 stores predistortion parameters for use in the predistortion processing the input signal. The calculated instantaneous power of the input signal of the digital baseband may be taken as an index number to retrieve corresponding predistortion parameter from the look-up table 115. The predistorter 100 predistorts the signal of the digital baseband according to the predistortion parameter to generate a predistorted signal. The D/A converter 101 performs a D/A conversion of the predistorted signal to generate an analog signal. The quadrature modulator 102 quadrature modulates the analog signal generated by the D/A converter 101, and then the mixer 103 converts the quadrature modulated signal into a RF signal. The amplifier 104 amplifies the RF signal from the mixer 103 and outputs the amplified signal (i.e., an output signal) to the antenna 105. The antenna 105 transmits the signal amplified by the amplifier 104.
The output signal of the amplifier 104 is partially fed back to the attenuator 107 through the coupler 106 coupled to an output end of the amplifier 104. The mixer 108 generates a baseband signal according to the signal attenuated by the attenuator 107. The analog band-pass filter 109 filters an outband signal from the baseband signal, and the power detector 110 calculates the instantaneous power of the filtered outband signal. The A/D converter 111 converts the instantaneous power of the outband signal output from the power detector 110 into a digital power signal. The averager 112 averages a plurality of digital power signals output from the A/D converter 111 to obtain an average digital power signal. The average digital power signal is input to the parameter updater 113 as the cost function. The parameter updater 113 updates parameters (u1, u2, ka) through an adaptive segment algorithm, wherein ka is a segment point, u1 and u2 are slopes on both sides of the segment point. The look-up table updater 114 updates the look-up table by using the updated parameter according to Equation (2):
                              L          ⁡                      (            k            )                          =                  {                                                                                                                u                      1                                        ×                                          (                                                                                                    L                            ′                                                    ⁡                                                      (                            k                            )                                                                          -                                                                              L                            ′                                                    ⁡                                                      (                            0                            )                                                                                              )                                                        +                                                            L                      ′                                        ⁡                                          (                      0                      )                                                                                                                    k                  ≤                                      k                    a                                                                                                                                                                  u                      2                                        ×                                          (                                                                                                    L                            ′                                                    ⁡                                                      (                            k                            )                                                                          -                                                                              L                            ′                                                    ⁡                                                      (                                                          k                              a                                                        )                                                                                              )                                                        +                                                            L                      ′                                        ⁡                                          (                                              k                        a                                            )                                                                                                                    k                  >                                      k                    a                                                                                                          (        2        )                            wherein, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table. The look-up table before updating may be an initial look-up table, e.g., a look-up table generated according to the transfer characteristic of the amplifier, or a look-up table updated in other method or updated previously.        
In the above adaptive segment predistortion method, the segment point ka is updated in an adaptive way. But when the number of the segment points is increased to achieve a better performance, a plurality of segment points need to be updated continuously, thus the convergence time of the adaptive digital predistortion device is increased. Particularly, when the variation of the transfer characteristic of the amplifier along with time elapse is to be compensated by the wireless communication device, excessive increase of the convergence time of the adaptive digital predistortion device cannot be accepted.
Thus, it requires an adaptive digital predistortion device and method capable of obtaining a good performance in an acceptable convergence time.